The present invention relates to a titanium oxide film having a blue color, which is formed on a surface of a metallic material used for metallic ornament such as a watch band, frames of eyeglasses and accessories, and ornaments for indoor use, so as to provide a blue exterior.
In order to impart blue metallic luster on a surface of the metallic material, a transparent oxide thin film which causes a blue interference color is formed on the surface thereof. The transparent oxide thin film is formed by ion plating, sputtering, chemical coloring process, sol-gel process, and others.
The blue produced by the interference color varies with the thickness of the film. In order to obtain a constant tone, the film must have a constant thickness. However, it is difficult to form such a thin film by the conventional method while precisely controlling the thickness thereof. In addition, the film is apt to assume other colors unless the film is precisely and evenly distributed. As a result, the tone of the film slightly varies at every production lot.
Moreover, the color varies in accordance with an angle at which the coated surface is seen. Namely, the surface may assume blue when viewed only from one direction, the front, for example, and assume another color when viewed from another direction.
Japanese Patent Application Laid-Open 63-161156 discloses another method of imparting blue to a metal surface. In accordance with the method, a mixture thin film which contains a mixture of titanium oxides such as titanium monoxide (TiO) and titanium trioxide (Ti.sub.2 O.sub.3) is precipitated by ion plating to form a blue thin film. Although the thickness of the film may not be precisely controlled, the surface assumes a uniform color. However, the obtained color is dark and lacks vividness.
The inventors of the present invention have made various researches on the mixture film and found that the type of the titanium oxides and the oxygen content in the film are important factors in determining the color of the film. The inventors filed Japanese Patent Application 2-270108 relating to a mixture film where average oxygen content therein is from 52 to 59 atom percent and the film comprises a mixture of titanium monoxide (TiO), titanium dioxide (TiO.sub.2) and other titanium oxides which are expressed as Ti.sub.n O.sub.2n-1 where n is an integer larger than 2. Neither the thickness of the film nor the viewing angle at which the film is viewed affects the color of the blue film.